1. Field of the Invention:
The invention relates to an agent for pre-treatment of rough mineral materials, containing swelling clays as impurities, intended for pigmentary applications in the form of aqueous suspensions with a dry matter content of at least 75% by weight, rheology stable over time and appropriate to each area of application.
The invention also relates to the new application of the pre-treatment agent in the preparation of suspensions of mineral materials containing swelling clays as impurities, which suspensions are meant to undergo exfoliation, abrasion and grinding actions while having a dry matter content of at least 75% by weight and a rheology appropriate to each application case.
2. Discussion of the Background:
The use of mineral materials such as carbonate, calcium silicate and sulfate as well as titanium dioxide, talc and kaolins for preparing industrial products intended for use in areas of painting, paper coating, fillers for rubber and synthetic resins, etc., has been known for a long time.
These mineral substances do not all possess a natural lamellate or layered structure facilitating their exfoliation, as is the case with certain substances such as the aluminum silicates commonly known by the name kaolin, and one must, therefore transform them by abrasion and/or grinding into a very fine aqueous suspension the constituent grains of which have the smallest size possible, i.e. less than a few microns, to use them as pigments.
The literature has described the importance and complexity of the techniques of abrasion or grinding of mineral substances in an aqueous medium to obtain a partially refined quality enabling a pigmentary application. Thus, in the special case of paper coating, a coating made up of mineral pigments such as kaolins, carbonate and calcium sulfate as well as titanium dioxide, suspended in water, and also containing dispersing and bonding agents as well as other adjuvants such as thickeners and coloring agents are well known.
In recent years, for paper coating, it has become desirable to have pigmentary suspensions with dry matter contents as high as possible in order to be able to improve the paper's size stability, drying conditions and, especially, the physical properties of the layer laid on the substrate, such as brilliance, for example.
To achieve the goal of a higher dry matter content in suspensions of mineral pigments intended for coating, a method was proposed, which consisted of using aqueous suspensions of mineral materials with rough granulometry.
Such a suspension is described, for example, in British Pat. No. 1,204,511 and is accomplished by dispersing chalk whiting, in an aqueous phase, containing 75 wt. % particles smaller than 2 microns, at a 70 to 85 wt. % dry matter concentration in the presence of a dispersing agent which is a sodium salt of polyacrylic acid.
To obtain a chalk whiting with the aforementioned granulometry, it is necessary to do either a dry or a wet grinding. If a dry grinding is done, the mineral materials ground must undergo a selection in order to eliminate as much as possible all particles that are too large.
If a wet grinding is done, the suspension resulting from said grinding is naturally limited to a dry matter concentration of at most 70% by weight due to the increase in viscosity. Thus, the suspension is then subjected to a partial drying to increase the dry matter concentration, or to a total drying, the disadvantage of which is that it causes a partial reagglomeration of the particles, forcing one to perform a size selection on the dried mineral materials.
As the method of obtaining a higher dry matter content in suspensions of mineral pigments meant for coating led to long, complex and therefore costly processes, a second method for achieving this end was proposed.
This second method consisted of using increasingly elaborate grinding agents during the exfoliation or wet grinding operations, facilitating the grinding operation, the dispersion of particles and the maintenance of suspension viscosities at acceptable values.
One type of grinding agent, described in French Pat. No. 2,488,814, composed of an alkaline acrylic polymer and/or copolymer with an average specific viscosity of less than 0.8, is made up of the fraction of this polymer and/or copolymer isolated by means of a polar solvent with a specific viscosity of from 0.3 to 0.8.
This type of grinding agent was essentially used to obtain a pigmentary suspension with a low viscosity and which was stable over time. The agent promoted the increase to 76% by weight of the dry matter content without being able to exceed this threshold.
A second type of dispersing agent, described in French Pat. No. 2,387,911, is composed of a salt of maleic and acrylic acid copolymers with an average molecular weight of from 2,000 to 10,000, making it possible to make pigmentary suspensions of calcium carbonate with a dry matter content of only from 40 to 60% by weight and unable to exceed the higher value.
A third type of grinding agent, described in French Pat. No. 2,531,444, is comprised of acid acrylic polymers and/or copolymers partially neutralized by at least one neutralization agent having at least one monovalent function. Such a grinding agent has made it possible to stabilize the rheologic properties of slurries over time but has not made it possible to increase the dry matter content of these suspensions past the threshold of 76% by weight.
Thus, thrugh the patent literature, the prior art has advocated numerous means of using rough materials during the grinding operation. These means, which were proposed to increase the dry matter concentration of the pigmentary suspensions, simultaneously to lower the viscosity of the suspensions during grinding and finally to prevent the viscosity from increasing over time, e.g. during storage, where shown to be ineffective. Although in many cases favorable results were obtained, poor results were observed when the rough mineral materials meant for pigmentary applications contained swelling clays, even in very low quantity. In this case, the means advocated by the prior art were inoperative with regard to even a slight increase in the dry matter concentrations of the aqueous suspensions subjected to abrasion or grinding.
Moreover, the means advocated often caused a simultaneous increase in the viscosity of these suspensions, this increase causing a mediocre refining of the mineral materials, a difficult manipulation of the pigmentary suspensions in their subsequent applications and sometimes an increase in mass of the suspensions after their grinding.